System and methods for performing dynamic pedicle integrity assessments

ABSTRACT

The present invention involves systems and related methods for performing dynamic pedicle integrity assessments involving the use of neurophysiology.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 11/061,184filed on Feb. 18, 2005 by Miles et al., now issued as U.S. Pat. No.7,657,308, which is a continuation of PCT Patent Application Serial No.PCT/US2004/025550 filed on Aug. 5, 2004 and published as WO/2005013805,which claims priority to: (1) U.S. Provisional Patent Application No.60/493,024 entitled “Systems and Methods for Performing PedicleIntegrity Assessments During Spinal Surgery” and filed on Aug. 5, 2003,and (2) U.S. Provisional Patent Application No. 60/540,083 entitled“Insulation Sheath for Medical Instruments and Related Methods” andfiled on Jan. 28, 2004. The entire contents of these earlierapplications are hereby expressly incorporated by reference into thisdisclosure as if set forth fully herein.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to a system and methods generally aimed atsurgery. More particularly, the present invention is directed at asystem and related methods for performing dynamic pedicle integrityassessments involving the use of neurophysiology.

II. Description of Related Art

A trend in spinal surgery is toward performing surgery in a minimallyinvasive or minimal access fashion to avoid the trauma of so-called openor “direct access” procedures. A specific area of interest is in theplacement of pedicle screws (percutaneous and open), which are typicallyemployed to effect posterior fixation in spinal fusion procedures. Whilegreat strides are being made in this area, a risk exists that thepedicle may become breached, cracked, or otherwise compromised due tothe formation and/or preparation of the pilot hole (designed to receivea pedicle screw) and/or due to the introduction of the pedicle screwinto the pilot hole. If the pedicle (or more specifically, the cortex ofthe medial wall, lateral wall, superior wall and/or inferior wall) isbreached, cracked, or otherwise compromised, the patient may experiencepain or neurologic deficit due to unwanted contact between the pediclescrew and exiting nerve roots. This oftentimes necessitates revisionsurgery, which is disadvantageously painful for the patient and costly,both in terms of recovery time and hospitalization.

Various attempts have been undertaken at performing pedicle integrityassessments. As used herein, the term “pedicle integrity assessment” isdefined as detecting or otherwise determining whether a part of apedicle has been breached, cracked, or otherwise compromised due to theformation and/or preparation of the pilot hole (designed to receive apedicle screw) and/or due to the introduction of the pedicle screw intothe pilot hole. “Formation” is defined as the act of creating an initialpilot hole in a pedicle, such as through the use of a drill or otherhole-forming element. “Preparation” is defined as the act of refining orotherwise acting upon the interior of the pilot hole to further prepareit to receive a pedicle screw, such as by introducing a tap or reamerelement into the initial pilot hole. “Introduction” is defined as theact of inserting or otherwise placing a pedicle screw into the initiallyformed and/or prepared pilot hole, such as by screwing the pedicle screwinto the pilot hole via a screw driver or similar element.

Among the attempts, X-ray and other imaging systems have been employed,but these are typically quite expensive and are oftentimes limited interms of resolution such that pedicle breaches may fail to be detected.

Still other attempts involve capitalizing on the insulatingcharacteristics of bone (specifically, that of the walls of the pedicle)and the conductivity of the exiting nerve roots themselves. That is, ifa wall of the pedicle is breached, a stimulation signal applied to thepedicle screw and/or the pilot hole (prior to screw introduction) willcause the various muscle groups coupled to the exiting nerve roots tocontract. If the pedicle wall has not been breached, the insulatingnature of the pedicle will prevent the stimulation signal frominnervating the given nerve roots such that the associated muscle groupswill not twitch. Traditional EMG monitoring systems may be employed toaugment the ability to detect such innervation. A drawback with suchprior art systems is that they do not lend themselves to assessingpedicle integrity in a dynamic fashion (that is, during the formation,preparation and/or introduction stages of pedicle screw fixation,whether in open or percutaneous procedures). A similar drawback exists,particularly in open cases, where fluids (e.g. blood and/or interstitialfluid) at or near the pedicle target site may cause shunting as thestimulation signal is applied during the formation, preparation and/orintroduction stages of pedicle screw fixation.

The present invention is directed at addressing this need andeliminating, or at least reducing, the effects of the shortcomings ofthe prior art as described above.

SUMMARY OF THE INVENTION

The present invention overcomes the drawbacks of the prior art byproviding, according to one broad aspect of the present invention, amethod for performing pedicle integrity assessments, comprising thesteps of: (a) establishing electrical communication between astimulation element and an interior of a pedicle hole during at leastone of pilot hole formation, pilot hole preparation, and pedicle screwintroduction; (b) applying a stimulation signal to said stimulationelement; and (c) monitoring to assess whether nerves adjacent saidpedicle are innervating as a result of the step of applying saidapplication of stimulation signal to said stimulation element.

The present invention overcomes the drawbacks of the prior art byproviding, according to another broad aspect of the present invention, asystem for performing pedicle integrity assessments comprising a medicalinstrument for use in at least one of pedicle hole formation, holepreparation, and pedicle screw insertion at a pedicle target site, saidpedicle target site in the general location of neural structures. Asensor is configured to detect a voltage response from musclesassociated with said neural structures. A control unit is coupled tosaid medical instrument and the sensor, the control unit beingconfigured to (a) transmit a stimulation signal to said medicalinstrument, (b) receive a voltage response from the sensor, and (c)determine whether said neural structures are innervating as a result ofthe step of applying said application of stimulation signal to saidmedical instrument element.

But for the systems and methods of the present invention, patients maybe released and subsequently experience pain and/or neurologic deficitdue to unwanted contact between the exiting nerve root and misplacedpedicle screws, which oftentimes requires another costly and painfulsurgery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary surgical system 20 capableof performing dynamic pedicle integrity assessments according to thepresent invention;

FIG. 2 is a block diagram of the surgical system 20 shown in FIG. 1;

FIG. 3 is a side view illustrating the use of first and second exemplarysystems for assessing pedicle integrity according to the presentinvention;

FIG. 4 is a side view illustrating the use of third and fourth exemplarysystems for assessing pedicle integrity according to the presentinvention;

FIG. 5 is a side view illustrating the use of a fifth and sixthexemplary system for assessing pedicle integrity according to thepresent invention;

FIG. 6 is a perspective view of the first exemplary system for assessingpedicle integrity according to the present invention as shown in FIG. 3,comprising a K-wire insulator electrically coupled to a handle assembly;

FIG. 7 is a perspective view of the third exemplary system for assessingpedicle integrity according to the present invention as shown in FIG. 4,comprising a universal insulating assembly including a handle assemblycoupled to an insulating cannula according to the present invention;

FIG. 8 is a perspective view illustrating an exemplary electricalcoupling mechanism capable of being disposed within the handle assemblyshown in FIG. 7;

FIGS. 9-11 are perspective views illustrating insulating cannulas ofvarying sizes and dimensions for use with the handle assembly of FIG. 7according to the present invention;

FIG. 12 is a perspective view of the fifth exemplary system forassessing pedicle integrity according to the present invention as shownin FIG. 5, comprising a universal coupling assembly having aspring-loaded contact plunger and an electrical cable for electricallyconnecting the contract plunger to the handle assembly;

FIGS. 13A-13C are top and side views of the universal coupling assemblyof FIG. 5, illustrating the universal coupling assembly by itself (FIG.13A) and coupled to an exemplary tool (FIGS. 13B and 13C);

FIGS. 14-16 are side views of the sixth exemplary sixth system forassessing pedicle integrity according to the present invention as shownin FIG. 5, comprising an insulating sheath dimensioned to be used in anopen procedure to prevent current shunting between an instrumentdelivering electrical stimulation and the surrounding tissues and/orfluids;

FIG. 17 is a graph illustrating a plot of a stimulation current pulsecapable of producing a neuromuscular response (EMG) of the type shown inFIG. 18;

FIG. 18 is a graph illustrating a plot of the neuromuscular response(EMG) of a given myotome over time based on a current stimulation pulse(such as shown in FIG. 17) applied to a nerve bundle coupled to thegiven myotome;

FIG. 19 is an illustrating (graphical and schematic) of a method ofautomatically determining the maximum frequency (F_(Max)) of thestimulation current pulses according to one embodiment of the presentinvention;

FIG. 20 is a graph illustrating a plot of EMG response peak-to-peakvoltage (Vpp) for each given stimulation current level (I_(Stim))forming a stimulation current pulse according to the present invention(otherwise known as a “recruitment curve”);

FIG. 21 is a graph illustrating a traditional stimulation artifactrejection technique as may be employed in obtaining each peak-to-peakvoltage (Vpp) EMG response according to the present invention;

FIG. 22 is a graph illustrating the traditional stimulation artifactrejection technique of FIG. 21, wherein a large artifact rejectioncauses the EMG response to become compromised;

FIG. 23 is a graph illustrating an improved stimulation artifactrejection technique according to the present invention;

FIG. 24 is a graph illustrating an improved noise artifact rejectiontechnique according to the present invention;

FIG. 25 is a graph illustrating a plot of a neuromuscular response (EMG)over time (in response to a stimulus current pulse) showing the mannerin which voltage extrema (V_(Max or Min)), (V_(Min or Max)) occur attimes T1 and T2, respectively;

FIG. 26 is a graph illustrating a histogram as may be employed as partof a T1, T2 artifact rejection technique according to an alternateembodiment of the present invention;

FIGS. 27A-27E are graphs illustrating a current threshold-huntingalgorithm according to one embodiment of the present invention;

FIG. 28 is a series of graphs illustrating a multi-channel currentthreshold-hunting algorithm according to one embodiment of the presentinvention;

FIGS. 29-30 are exemplary screen displays illustrating one embodiment ofthe pedicle integrity assessment feature of the present invention; and

FIGS. 31-33 are exemplary screen displays illustrating anotherembodiment of the pedicle integrity assessment feature of the presentinvention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. The systems disclosed herein boast a variety ofinventive features and components that warrant patent protection, bothindividually and in combination.

The present invention is directed at performing dynamic pedicleintegrity assessments. In its most fundamental form, the presentinvention involves establishing electrical communication between astimulation source and the interior of a pedicle hole during the holeformation, hole preparation, and/or screw introduction steps of pediclescrew fixation. By applying a stimulation signal (such as thecurrent-controlled signal described below) during these steps, andmonitoring the neuro-muscular responses resulting from this stimulation,the system of the present invention can automatically detect andcommunicate to the user whether the integrity of the pedicle has beencompromised during to the steps of hole formation, hole preparationand/or screw introduction. In so doing, the present inventionadvantageously allows the surgeon to immediate appreciate the breach orpotential breach of the pedicle and correct the screw placement. Thisavoids the problem of patients being released only to subsequentlyexperience pain and/or neurologic deficit due to unwanted contactbetween the exiting nerve root and misplaced pedicle screws.

Although shown and described within the context of a particularexemplary system having a stimulation source and monitoring capacity, itwill be appreciated by those skilled in the art that any number ofsystems for providing a stimulation signal and for monitoring to assesspedicle breach may be employed without departing from the scope of thepresent invention.

FIGS. 1-2 illustrate, by way of example only, a surgical system 20provided in accordance with a broad aspect of the present invention. Thesurgical system 20 includes a control unit 22, a patient module 24, anEMG harness 26 and return electrode 28 coupled to the patient module 24,and a host of pedicle screw test accessories 30 capable of being coupledto the patient module 24 via an accessory cable 32 in combination with ahandle assembly 36. In the embodiment shown, the pedicle screw testaccessories 30 include (by way of example only) a K-wire insulator 34, auniversal insulating assembly 38, and a universal electrical coupler 35.As will be described in greater detail below, a K-wire 37 and a tapmember 39 are shown, by way of example, as exemplary stimulationelements according to the present invention. The K-wire 37 may beelectrically coupled to the control unit 22 and/or patient module 24 (soas to receive a stimulation signal) through the use of the K-wireinsulator 34, the universal insulating assembly 38 and/or the electricalcoupler 35 (provided the K-wire 37 is insulated in some manner if usedin percutaneous procedure). The tap member 39 may be electricallycoupled to the control unit 22 and/or patient module 24 (so as toreceive a stimulation signal) through the use of the universalinsulating assembly 38, the electrical coupler 35 (provided the tapmember 39 is insulated in some manner if used in a percutaneousprocedure) and/or by bringing a stimulation element into contact withthe tap member 39, such as by (for example) providing a longitudinalcannulation within the tap member 39 and disposing an electricallycoupled K-wire 37 therein.

The control unit 22 includes a touch screen display 40 and a base 42,which collectively contain the essential processing capabilities forcontrolling the surgical system 20. The patient module 24 is connectedto the control unit 22 via a data cable 44, which establishes theelectrical connections and communications (digital and/or analog)between the control unit 22 and patient module 24. The main functions ofthe control unit 22 include receiving user commands via the touch screendisplay 40, activating stimulation, processing signal data according todefined algorithms (described below), displaying received parameters andprocessed data, and monitoring system status and reporting faultconditions. The touch screen display 40 is preferably equipped with agraphical user interface (GUI) capable of communicating information tothe user and receiving instructions from the user. The display 40 and/orbase 42 may contain patient module interface circuitry that commands thestimulation sources, receives digitized signals and other informationfrom the patient module 24, processes the EMG responses to extractcharacteristic information for each muscle group, and displays theprocessed data to the operator via the display 40.

As will be described in greater detail below, the surgical system 20 iscapable of performing pedicle integrity assessments in a dynamicfashion, that is, during the formation and/or preparation of the pilothole and/or during pedicle screw placement. Surgical system 20accomplishes this by having the control unit 22 and patient module 24cooperate to send stimulation signals to one or more stimulationelectrodes or electrode regions on the various pedicle screw testaccessories 30. Depending upon effect of pilot hole formation, pilothole preparation and/or pedicle screw introduction (namely, on the boneforming the pedicle), the stimulation signals may cause nerves adjacentto or in the general proximity of the K-wire 37 and/or tap member 39 toinnervate, which, in turn, can be monitored via the EMG harness 26. Thepedicle integrity assessment feature of the present invention are basedon assessing the evoked response of the various muscle myotomesmonitored by the surgical system 20 via EMG harness 26.

The accessory handle assembly 36 includes a cable 55 for establishingelectrical communication with the patient module 24 (via the accessorycable 32). In a preferred embodiment, each pedicle screw test accessory30 (namely, K-wire insulator 34, universal insulating assembly 38, andelectrical coupler 35) includes a proximal electrical connector 56, adistal electrical connector (described below), and an electrical cable57 extending therebetween. The proximal electrical connector 56 ispreferably threaded and designed to engage with the distal end 59 of thehandle assembly 36. In this fashion, the screw test accessories 30 maybe quickly and easily coupled (electrically and mechanically) to theaccessory handle assembly 36. The distal electrical connector of theK-wire insulator 34 and universal insulating assembly 38 may compriseany number of suitable mechanisms for establishing electricalcommunication with an instrument passing therethrough (such as a K-wire37 passing through the K-wire insulator 34 and/or the universalinsulating assembly 38, and such as a tap member 39 extending throughthe universal insulating assembly 38). In a preferred embodiment, thedistal electrical connectors within the universal insulating assembly 38will be capable of expanding, moving or otherwise accommodatinginstruments of varying diameters according to the present invention. Thedistal electrical connector of the coupler 35 may include any number ofsuitable electrode or electrode regions (including protrusions) on orabout the distal (or pinching) ends of the clamp arms 61 forming thecoupler 35. Corresponding regions (such as electrodes or electroderegions—including indentations) may be provided on the K-wire 37, thetap member 39, such as where such devices are to be directly coupled tothe handle assembly 36 (i.e. where K-wire 37 and/or tap member 39 aredisposed through insulating elements that do not include distalelectrical connectors, for percutaneous procedures) according to thepresent invention.

In all situations, the user may operate one or more buttons of thehandle assembly 36 to selectively initiate a stimulation signal(preferably, a current signal) from the patient module 24 to the pedicleprobe 56. According to the present invention, this stimulation signal isapplied with the K-wire 37 and/or tap member 39 touching the interiorwall during the formation and/or preparation of the pilot hole and/orwith the K-wire 37 touching a pedicle screw during introduction. Thisserves to test the integrity of the wall(s) of the pedicle. That is, abreach or compromise in the integrity of the pedicle will allow thestimulation signal to pass through the pedicle and innervate an adjacentnerve root. By monitoring the myotomes associated with the nerve roots(via the EMG harness 26 and recording electrode 27) and assessing theresulting EMG responses (via the control unit 22), the surgical system20 can assess whether a pedicle breach occurred during hole formation,preparation and/or screw introduction. If a breach or potential breachis detected, the user may simply withdraw the misplaced pedicle screwand redirect to ensure proper placement.

FIG. 3 illustrates two exemplary manners of performing pedicle integrityassessments according to the present invention (both percutaneous by wayof example), one employing the K-wire insulator 34 and one employing theelectrical coupler 35. With combined reference with FIG. 6, the K-wireinsulator 34 according to the present invention includes an elongateinsulating body 60 having a tapered distal end 63, open distal andproximal ends, and a lumen or cannulation extending therebetweendimensioned to receive and pass the K-wire 37. A cap element 64 isprovided for placement in the proximal end of the insulating body 60.The cap element 64 has a lumen therewithin dimensioned to pass theK-wire 37 and includes the distal electrical connector (not shown)coupled to the electrical cable 57. As shown in FIG. 3, the K-wireinsulator 34 may be advanced to the pedicle target site in apercutaneous fashion, by either establishing a virgin approach to thepedicle target site or by passing through a previously establishedpercutaneous corridor (such as may be left or formed by commerciallyavailable percutaneous pedicle screw placement systems). This processmay be facilitated by first establishing a pilot hole through the use ofa so-called Jam-Sheede needle (comprising an inner rigid needle elementdisposed within a rigid outer needle element), after which point theinner rigid needle element is removed such that the K-wire 37 may beintroduced into the pilot hole. The outer rigid needle element of theJam-Sheede device may then be removed, leaving the K-wire 37 in place.The K-wire insulator 34 may then be advanced over the K-wire 37. Oncethe distal end 63 of the K-wire insulator 34 abuts the opening of thepedicle pilot hole, buttons 64 on the handle member 36 may be employedto apply the stimulation signal to the K-wire 37. In this fashion, themajority of the K-wire 37 is insulated from the surrounding tissue,while the distal end of the K-wire 37 may be brought into direct contactwith the pilot hole to perform pedicle integrity assessments accordingto one embodiment of the present invention. As will be appreciated, thissame technique could be employed to bring the stimulation electrode orelectrode region of the K-wire 37 into contact with a portion of a fullyinserted pedicle screw (not shown). As will also be appreciated, theouter rigid needle element of a Jam-Sheede device may itself beinsulated, such that the K-wire 37 may be disposed through theJam-Sheede needle and still perform according to the present invention.

FIG. 3 also illustrates that the electrical coupler 35 may be employedto perform pedicle integrity assessments, by way of example only, byestablishing electrical communication between the tap member 39 duringpreparation of the pedicle hole. The electrical coupler 35 accomplishesthis by engaging the electrode or electrode regions on the opposingclamping arms 61 against a portion of the proximal end of the tap member39. To facilitate this, the tap member 39 may be equipped withindentations or similar features for matingly engaging withcorresponding features on the distal regions of the clamping arms 61. Inthe embodiment shown, an insulated cannula 66 is provided for insulatingall but the exposed distal and proximal ends of the tap member 39 (dueto the percutaneous approach shown by way of example). As with the body60 of the K-wire insulator 34, the insulated cannula 66 is preferablyequipped with a tapered distal end 67. In use, the tap member 39 will beadvanced through the insulated cannula 66 (such as by being passed overa K-wire 37 via an internal cannulation) and rotated to prepare threadsalong the interior of the pilot hole. During preparation of the pilothole in this manner, the handle member 36 may be used to apply thestimulation signal to the electrical coupler 35 which, in turn,transmits this stimulation signal to the interior of the pilot hole toperform pedicle integrity assessments according to another embodiment ofthe present invention. If the pedicle has not been breached during thepreparation process, the tap member 39 may then be removed and a pediclescrew introduced into the prepared pilot hole. By selecting a pediclescrew having the same approximate characteristics (i.e. pitch, threadheight, diameter, length, etc. . . . ) as the tapping (distal) portionof the tap member 39, the need to perform further pedicle integrityassessments during the introduction of the pedicle screw may be obviatedor minimized.

FIG. 4 illustrates two more exemplary manners of performing pedicleintegrity assessments according to the present invention (bothpercutaneous by way of example), one employing the universal insulatingassembly 38 and one employing the electrical coupler 35. With combinedreference to FIGS. 7-11, the universal insulating assembly 38 includes ahandle assembly 68 and an insulated cannula 70 extending from the distalportion of the handle assembly 68. As best seen in FIG. 7, the handleassembly 68 includes a housing member 71 and an electrical connectorport 72 for connection with the electrical cable 57. With reference toFIG. 8, the housing member 71 contains a universal electrical couplingmechanism 73 comprising, by way of example, a plurality of contactelements 74 (in this case springs extending between posts 75). A lumen76 is provided (by way of example only) in the approximate center of(and extending between) upper and lower base members 77. The contactelements 74 are positioned in a transverse fashion such that theyintersect generally in the same plane as the center of the lumen 76. Inthis fashion, any metallic or conductive instrument passed through thelumen 76 will be brought into contact with the contact elements 74,thereby providing the ability to apply an electrical signal to theinstrument. Moreover, the contact elements 74 are capable of moving,expanding, or otherwise accommodating instruments having a variety ofdiameters. As best shown in FIGS. 9-11, the insulated cannula 70 may beprovided having any number of different lengths and widths, dependingupon the device to be passed through it. A threaded base member 78 ispreferably coupled to each insulated cannula 70 to facilitate couplingthe particular insulated cannula 70 to a corresponding threaded portionon the distal region of the housing member 71. In this fashion, asurgeon may quickly and easily change between any of a variety ofinsulating cannulas 70 depending upon the application (i.e. depth to thepedicle target site) and the device to be passed therethrough (i.e. thetap member 39 as shown in FIG. 4).

The insulating cannula 70 serves to isolate a portion of the instrumentas it is passed through the handle assembly 68. In this fashion, theinsulating cannula 70 may be advanced to a pedicle target site, such asto the opening of a pedicle pilot hole as shown in FIG. 4. Although notshown, it is to be readily appreciated that the present invention alsocontemplates advancing the distal end of the insulating cannula 70 overor in general abutment with a proximal portion of a percutaneouslyplaced pedicle screw pedicle screw. In either instance, an instrument ordevice (such as, by way of example, K-wire 37 or the tap member 39,depending upon the situation) may be passed through the handle member 68during the steps of hole formation, hole preparation and/or screwintroduction. The insulating cannulas 70 are of varying size dependingupon the particular target site and surgical application, but maypreferably be provided ranging from 0 inches to 24 inches in length andof any diameter suitable to pass the instrument of interest.

FIG. 4 also illustrates a variant of the embodiment shown in FIG. 3,except that the insulated cannula 66 is specifically dimensioned to passthe K-wire 37, as opposed to larger diameter instruments such as the tapmember 39 as shown in FIG. 3. In this instance, the electrical coupler35 may be used to establish electrical communication between the K-wire37 and the interior of a pedicle hole. With the distal end of the K-wire37 in such electrical communication with the interior of the pilot hole,the handle assembly 36 may be employed to apply the stimulation signalto perform a pedicle integrity assessment according to the presentinvention. Placement of the K-wire 37 within the pilot hole, and theadvancement of the insulated cannula 66, may be the same as describedabove with reference to the Jam-Sheede device described above.

FIG. 5 illustrates an alternate embodiment of the universal electricalcoupler 35 shown above (designated 35′ for clarity), employed in both apercutaneous procedure (on the left) and an open procedure (on theright). With brief reference to FIGS. 12-13, the electrical coupler 35′comprises a contact plunger 41 disposed within a housing member 43. Theplunger 41 is generally cylindrical in shape and includes an electricalcontact region 45 at its distal end, as best viewed in FIGS. 13A-13C.The electrical contact region 45 is electrically coupled to theelectrical cable 57, which extends from the proximal end of the plunger41 (through a hole in the proximal end of the housing member 43). Theplunger 41 is preferably spring-loaded relative to the housing member 43such that it is normally biased into a closed position (best seen inFIG. 13A). The housing member 43 includes a generally cylindrical body47 (dimensioned to slideably receive part or all of the plunger 41therein) and a distal arm 49. The distal arm 49 includes a generallyarcuate interior surface 51, and the electrical contact region 45includes a generally arcuate surface 53 (both viewed best in FIG. 13A).

To use the coupler 35′, a user need only pull on the electrical cable 57(which may be reinforced and/or provided with a strain-relief asnecessary) to retract the plunger 41, thereby opening a region betweenthe surface 51 of arm 49 and surface 53 of electrical contact region 45.With the plunger 41 in this retracted state, any number of generallycylindrical devices (as shown generically at 65 in FIGS. 13B-13C) may bepositioned within this open region. The plunger 41 may thereafter bereleased, which under spring-loading pushes the electrical contactregion 45 of the plunger 41 into abutment (and electrical communication)with the device 65. It will be appreciated that device 65 may compriseany number of suitable instruments for use in the hole formation, holepreparation and/or screw introduction processes of pedicle screwfixation (such as, by way of example only, the K-wire 37 and/or the tap39 shown in FIG. 5).

Returning to FIG. 5, once the electrical coupler 35′ of the presentinvention is coupled to the particular instrument, such as the K-wire 37and/or the tap 39, it may be used as follows in accordance with thepresent invention. In the percutaneous procedure, the electrical coupler35′ is employed to establish electrical communication between the tapmember 39 during preparation of the pedicle hole. In the embodimentshown, an insulated cannula 66 is provided for insulating all but theexposed distal and proximal ends of the tap member 39 (due to thepercutaneous approach shown by way of example). As with the body 60 ofthe K-wire insulator 34, the insulated cannula 66 is preferably equippedwith a tapered distal end 67. In use, the tap member 39 will be advancedthrough the insulated cannula 66 (such as by being passed over a K-wire37 via an internal cannulation) and rotated to prepare threads along theinterior of the pilot hole. During preparation of the pilot hole in thismanner, the handle member 36 may be used to apply the stimulation signalto the electrical coupler 35′ which, in turn, transmits this stimulationsignal to the interior of the pilot hole during preparation to performpedicle integrity assessments according to another embodiment of thepresent invention. If the pedicle is not breached as preparationprogresses, then tap member 39 may be removed and a pedicle screwintroduced into the prepared pilot hole. By selecting a pedicle screwhaving the same approximate characteristics (i.e. pitch, thread height,diameter, length, etc. . . . ) as the tapping (distal) portion of thetap member 39, the need to perform further pedicle integrity assessmentsduring the introduction of the pedicle screw may be obviated orminimized.

In an open procedure (or minimally invasive relative to the percutaneousapproach described above), the electrical coupler 35′ may—according to afurther aspect of the present invention—be employed an insulating sheath110. With brief reference to FIGS. 14-16, the insulation sheath 110includes an elongated sheath 120 and a molded expandable seal-tip 130,which collectively provide the ability to insulate an electrifiedmedical instrument (e.g. the K-wire 37 in FIG. 5 or the awl 180 in FIGS.14-16) so as to avoid unwanted shunting according to this aspect of thepresent invention. To accomplish this, a continuous lumen is providedextending between a proximal opening 124 formed in the elongated sheath120 and a distal opening 136 formed in the seal tip 130. In use,described by way of example with reference to awl 180, the tip 186 ofthe awl 180 is advanced into the proximal opening 124, through the innerlumen, and out the distal opening 136 in the seal tip 130. Whenpositioned as such, all regions of the awl 180 (except the distal tip186) will be insulated from fluids (e.g. blood and/or interstitialfluid) during the hole formation step, with the shaft 184 disposedwithin the lumen of the sheath 110 and the handle 182 disposed outsidethe sheath 110. It will be appreciated that, while described withreference to an awl, the insulation sheath 110 of the present inventionmay be employed with any number of different instruments and may finduse during the steps of hole preparation and/or screw introduction aswell.

The elongated sheath 120 may be comprised of any material capable ofproviding a flexible layer of protection from bodily fluids andinsulation from electric current. By way of example only, elongatedsheath 120 may comprise a thin polyurethane film. Alternativeembodiments of elongated sheath 120 may be composed of a variety ofinsulative materials, including but not limited to rubber, plastic,resilient polymers, elastomers, polyesters, polyimides, silicicones,fluoropolymers, and teflon. Preferably, elongated sheath 120 is flexibleand/or compressible to accommodate different sizes of medicalinstruments and the areas of the body in which said instruments areused. Elongated sheath 120 may be made by seam welding at least one flatpolyurethane (or other suitable material) sheet to make an enclosedcorridor of various shapes. In such a case, elongated sheath 120 willinclude at least one seam along the length of the corridor. Preferably,the elongated sheath 120 is of sufficient length such that proximal end122 will begin to bunch up (due to its flexible character) once themedical instrument is fully inserted into sheath 110 (e.g. once thedistal tip of the instrument is protruding from distal opening 136).Distal opening 136 is preferably resilient yet pliable enough so that itwill remain generally flush against shaft of the medical instrument suchthat a seal preventing leakage of fluid inside sheath 110 is formed.

As noted above, the system 20 described generally above is exemplary ofa system including a stimulation source and monitoring capacity for usein performing pedicle integrity assessment according to the presentinvention. It will be appreciated by those skilled in the art, however,that any number of systems for providing a stimulation signal and formonitoring to assess pedicle breach may be employed without departingfrom the scope of the present invention. That said, the followingdiscussion elaborates on the particular algorithms and principles behindthe neurophysiology for performing pedicle integrity assessmentsaccording to the exemplary embodiment shown (system 20 of FIGS. 1-2)according to the present invention.

FIGS. 17 and 18 illustrate a fundamental aspect of the presentinvention: a stimulation signal (FIG. 17) and a resulting evokedresponse (FIG. 18). By way of example only, the stimulation signal ispreferably a stimulation current signal (I_(Stim)) having rectangularmonophasic pulses with a frequency and amplitude adjusted by systemsoftware. In a still further preferred embodiment, the stimulationcurrent (I_(Stim)) may be coupled in any suitable fashion (i.e. AC orDC) and comprises rectangular monophasic pulses of 200 microsecondduration. The amplitude of the current pulses may be fixed, but willpreferably sweep from current amplitudes of any suitable range, such asfrom 2 to 100 mA. For each nerve and myotome there is a characteristicdelay from the stimulation current pulse to the EMG response (typicallybetween 5 to 20 ms). To account for this, the frequency of the currentpulses is set at a suitable level such as, in a preferred embodiment, 4Hz to 10 Hz (and most preferably 4.5 Hz), so as to prevent stimulatingthe nerve before it has a chance to recover from depolarization. The EMGresponse shown in FIG. 18 can be characterized by a peak-to-peak voltageof V_(pp)=V_(max)−V_(min).

FIG. 19 illustrates an alternate manner of setting the maximumstimulation frequency, to the extent it is desired to do so rather thansimply selecting a fixed maximum stimulation frequency (such as 4.5 Hz)as described above. According to this embodiment, the maximum frequencyof the stimulation pulses is automatically adjusted. After eachstimulation, Fmax will be computed as: Fmax=1/(T2+T_(Safety Margin)) forthe largest value of T2 from each of the active EMG channels. In oneembodiment, the Safety Margin is 5 ms, although it is contemplated thatthis could be varied according to any number of suitable durations.Before the specified number of stimulations, the stimulations will beperformed at intervals of 100-120 ms during the bracketing state,intervals of 200-240 ms during the bisection state, and intervals of400-480 ms during the monitoring state. After the specified number ofstimulations, the stimulations will be performed at the fastest intervalpractical (but no faster than Fmax) during the bracketing state, thefastest interval practical (but no faster than Fmax/2) during thebisection state, and the fastest interval practical (but no faster thanFmax/4) during the monitoring state. The maximum frequency used untilF_(max) is calculated is preferably 10 Hz, although slower stimulationfrequencies may be used during some acquisition algorithms. The value ofF_(max) used is periodically updated to ensure that it is stillappropriate. For physiological reasons, the maximum frequency forstimulation will be set on a per-patient basis. Readings will be takenfrom all myotomes and the one with the slowest frequency (highest T2)will be recorded.

A basic premise behind the neurophysiology employed in the presentinvention is that each nerve has a characteristic threshold currentlevel (I_(Thresh)) at which it will depolarize. Below this threshold,current stimulation will not evoke a significant EMG response (V_(pp)).Once the stimulation threshold (I_(Thresh)) is reached, the evokedresponse is reproducible and increases with increasing stimulation untilsaturation is reached. This relationship between stimulation current andEMG response may be represented graphically via a so-called “recruitmentcurve,” such as shown in FIG. 20, which includes an onset region, alinear region, and a saturation region. By way of example only, thepresent invention defines a significant EMG response to have a Vpp ofapproximately 100 uV. In a preferred embodiment, the lowest stimulationcurrent that evokes this threshold voltage (V_(Thresh)) is calledI_(Thresh). As will be described in greater detail below, changes in thecurrent threshold (I_(Thresh)) may be indicative of a change in thedegree of electrical communication between a stimulation electrode and anerve. This is helpful in assessing if a screw or similar instrument hasinadvertently breached the cortex of a pedicle. More specifically, wherean initial determination of (I_(Thresh)), such as by applying astimulation current to the interior of a hole during formation and/orpreparation, is greater than a later determination of (I_(Thresh)), suchas by applying a stimulation current to the pedicle screw duringinsertion, the decrease in I_(Thresh), if large enough, may indicateelectrical communication between the pedicle screw and the nerve. Basedon the insulation properties of bone, such electrical communicationwould indicate a breach of the pedicle.

In order to obtain this useful information, the present invention mustfirst identify the peak-to-peak voltage (Vpp) of each EMG responsecorresponding a given stimulation current (I_(Stim)). The existencestimulation and/or noise artifacts, however, can conspire to create anerroneous Vpp measurement of the electrically evoked EMG response. Toovercome this challenge, the surgical system 20 of the present inventionmay employ any number of suitable artifact rejection techniques,including the traditional stimulation artifact rejection technique shownin FIG. 21. Under this technique, stimulation artifact rejection isundertaken by providing a simple artifact rejection window T1 _(WIN) atthe beginning of the EMG waveform. During this T1 window, the EMGwaveform is ignored and Vpp is calculated based on the max and minvalues outside this window. (T1 is the time of the first extremum (minor max) and T2 is the time of the second extremum.) In one embodiment,the artifact rejection window T1 _(WN) may be set to about 7.3 msec.While generally suitable, there are situations where this stimulationartifact rejection technique of FIG. 21 is not optimum, such as in thepresence of a large stimulation artifact (see FIG. 22). The presence ofa large stimulation artifact causes the stimulation artifact to crossover the window T1 _(WIN) and blend in with the EMG. Making thestimulation artifact window larger is not effective, since there is noclear separation between EMG and stimulation artifact.

FIG. 23 illustrates a stimulation artifact rejection technique accordingto the present invention, which solves the above-identified problem withtraditional stimulation artifact rejection. Under this technique, a T1validation window (T1−V_(WIN)) is defined immediately following the T1window (T1 _(WIN)). If the determined Vpp exceeds the threshold forrecruiting, but T1 falls within this T1 validation window, then thestimulation artifact is considered to be substantial and the EMG isconsidered to have not recruited. An operator may be alerted, based onthe substantial nature of the stimulation artifact. This method ofstimulation artifact rejection is thus able to identify situations wherethe stimulation artifact is large enough to cause the Vpp to exceed therecruit threshold. To account for noise, the T1 validation window(T1−V_(WIN)) should be within the range of 0.1 ms to 1 ms wide(preferably about 0.5 ms). The T1 validation window (T1−V_(WIN)) shouldnot be so large that the T1 from an actual EMG waveform could fallwithin.

FIG. 24 illustrates a noise artifact rejection technique according tothe present invention. When noise artifacts fall in the time windowwhere an EMG response is expected, their presence can be difficult toidentify. Artifacts outside the expected response window, however, arerelatively easy to identify. The present invention capitalizes on thisand defines a T2 validation window (T2−V_(WIN)) analogous to the T1validation window (T1−V_(WIN)) described above with reference to FIG.23. As shown, T2 must occur prior to a defined limit, which, accordingto one embodiment of the present invention, may be set having a range ofbetween 40 ms to 50 ms (preferably about 47 ms). If the Vpp of the EMGresponse exceeds the threshold for recruiting, but T2 falls beyond theT2 validation window (T2−V_(WIN)), then the noise artifact is consideredto be substantial and the EMG is considered to have not recruited. Anoperator may be alerted, based on the substantial nature of the noiseartifact.

FIG. 25 illustrates a still further manner of performing stimulationartifact rejection according to an alternate embodiment of the presentinvention. This artifact rejection is premised on the characteristicdelay from the stimulation current pulse to the EMG response. For eachstimulation current pulse, the time from the current pulse to the firstextremum (max or min) is T₁ and to the second extremum (max or min) isT₂. As will be described below, the values of T₁, T₂ are each compiledinto a histogram period (see FIG. 26). New values of T₁, T₂ are acquiredfor each stimulation and the histograms are continuously updated. Thevalue of T₁ and T₂ used is the center value of the largest bin in thehistogram. The values of T₁, T₂ are continuously updated as thehistograms change. Initially Vpp is acquired using a window thatcontains the entire EMG response. After 20 samples, the use of T₁, T₂windows is phased in over a period of 200 samples. Vmax and Vmin arethen acquired only during windows centered around T₁, T₂ with widths of,by way of example only, 5 msec. This method of acquiring V_(pp)automatically rejects the artifact if T₁ or T₂ fall outside of theirrespective windows.

Having measured each Vpp EMG response (as facilitated by the stimulationand/or noise artifact rejection techniques described above), this Vppinformation is then analyzed relative to the stimulation current inorder to determine a relationship between the nerve and the givenstimulation element transmitting the stimulation current. Morespecifically, the present invention determines these relationships(between nerve and the stimulation element) by identifying the minimumstimulation current (I_(Thresh)) capable of resulting in a predeterminedVpp EMG response. According to the present invention, the determinationof I_(Thresh) may be accomplished via any of a variety of suitablealgorithms or techniques.

FIGS. 27A-27E illustrate, by way of example only, a threshold-huntingalgorithm for quickly finding the threshold current (I_(Thresh)) foreach nerve being stimulated by a given stimulation current (I_(Stim)).Threshold current (I_(Thresh)), once again, is the minimum stimulationcurrent (I_(Stim)) that results in a Vpp that is greater than a knownthreshold voltage (V_(Thresh)). The value of is adjusted by a bracketingmethod as follows. The first bracket is 0.2 mA and 0.3 mA. If the Vppcorresponding to both of these stimulation currents is lower thanVThresh, then the bracket size is doubled to 0.2 mA and 0.4 mA. Thisdoubling of the bracket size continues until the upper end of thebracket results in a Vpp that is above VThresh. The size of the bracketsis then reduced by a bisection method. A current stimulation value atthe midpoint of the bracket is used and if this results in a Vpp that isabove VThresh, then the lower half becomes the new bracket.

Likewise, if the midpoint Vpp is below VThresh then the upper halfbecomes the new bracket. This bisection method is used until the bracketsize has been reduced to I_(Thresh) mA. I_(Thresh) may be selected as avalue falling within the bracket, but is preferably defined as themidpoint of the bracket.

The threshold-hunting algorithm of this embodiment will support threestates: bracketing, bisection, and monitoring. A stimulation currentbracket is a range of stimulation currents that bracket the stimulationcurrent threshold I_(Thresh). The width of a bracket is the upperboundary value minus the lower boundary value. If the stimulationcurrent threshold I_(Thresh) of a channel exceeds the maximumstimulation current, that threshold is considered out-of-range. Duringthe bracketing state, threshold hunting will employ the method below toselect stimulation currents and identify stimulation current bracketsfor each EMG channel in range.

The method for finding the minimum stimulation current uses the methodsof bracketing and bisection. The “root” is identified for a functionthat has the value −1 for stimulation currents that do not evokeadequate response; the function has the value +1 for stimulationcurrents that evoke a response. The root occurs when the function jumpsfrom −1 to +1 as stimulation current is increased: the function neverhas the value of precisely zero. The root will not be known exactly, butonly with a level of precision related to the minimum bracket width. Theroot is found by identifying a range that must contain the root. Theupper bound of this range is the lowest stimulation current I_(Thresh)where the function returns the value +1, i.e. the minimum stimulationcurrent that evokes response. The lower bound of this range is thehighest stimulation current I_(Thresh) where the function returns thevalue −1, i.e. the maximum stimulation current that does not evoke aresponse.

The pedicle integrity assessment function may begin by adjusting thestimulation current until the root is bracketed (FIG. 27B). The initialbracketing range may be provided in any number of suitable ranges. Inone embodiment, the initial bracketing range is 0.2 to 0.3 mA. If theupper stimulation current does not evoke a response, the upper end ofthe range should be increased. The range scale factor is 2. Thestimulation current should preferably not be increased by more than 10mA in one iteration. The stimulation current should preferably neverexceed the programmed maximum stimulation current. For each stimulation,the algorithm will examine the response of each active channel todetermine whether it falls within that bracket. Once the stimulationcurrent threshold of each channel has been bracketed, the algorithmtransitions to the bisection state.

During the bisection state (FIGS. 27C and 27D), threshold hunting willemploy the method described below to select stimulation currents andnarrow the bracket to a selected width (for example, 0.1 mA) for eachEMG channel with an in-range threshold. After the minimum stimulationcurrent has been bracketed (FIG. 27B), the range containing the root isrefined until the root is known with a specified accuracy. The bisectionmethod is used to refine the range containing the root. In oneembodiment, the root should be found to a precision of 0.1 mA. Duringthe bisection method, the stimulation current at the midpoint of thebracket is used. If the stimulation evokes a response, the bracketshrinks to the lower half of the previous range. If the stimulationfails to evoke a response, the bracket shrinks to the upper half of theprevious range. The proximity algorithm is locked on the electrodeposition when the response threshold is bracketed by stimulationcurrents separated by the selected width (i.e. 0.1 mA). The process isrepeated for each of the active channels until all thresholds areprecisely known. At that time, the algorithm enters the monitoringstate.

After identifying the threshold current I_(Thresh), this information maybe employed to determine any of a variety of relationships between thescrew test accessory and the nerve. For example, as will be described ingreater detail below, when determining the current threshold I_(thresh)of a nerve during pedicle integrity assessment, the relationship betweenthe pedicle testing assembly 36 and the nerve is whether electricalcommunication is established therebetween. If electrical communicationis established, this indicates that the medial wall of the pedicle hasbeen cracked, stressed, or otherwise breached as a result of pilot holeformation, pilot hole preparation, and/or screw introduction. If not,this indicates that the integrity of the medial wall of the pedicle hasremained intact. This characteristic is based on the insulatingproperties of bone.

In a significant aspect of the present invention, the relationshipsdetermined above based on the current threshold determination may becommunicated to the user in an easy to use format, including but notlimited to, alpha-numeric and/or graphical information regarding pedicleintegrity assessments, stimulation level, EMG responses, instrument inuse, set-up, and related instructions for the user. This advantageouslyprovides the ability to present simplified yet meaningful data to theuser, as opposed to the actual EMG waveforms that are displayed to theusers in traditional EMG systems. Due to the complexity in interpretingEMG waveforms, such prior art systems typically require an additionalperson specifically trained in such matters which, in turn, can bedisadvantageous in that it translates into extra expense (having yetanother highly trained person in attendance) and oftentimes presentsscheduling challenges because most hospitals do not retain suchpersonnel.

When employed in lumbar spinal procedures, for example, such EMGmonitoring would preferably be accomplished by connecting the EMGharness 26 to the myotomes in the patient's legs corresponding to theexiting nerve roots associated with the particular spinal operationlevel. (This may similarly be performed during cervical spinalprocedures by employing the system of the present invention to monitorthe myotomes on the arms of the patient). In a preferred embodiment,this is accomplished via 8 pairs of EMG electrodes 27 placed on the skinover the major muscle groups on the legs (four per side), an anodeelectrode 29 providing a return path for the stimulation current, and acommon electrode 31 providing a ground reference to pre-amplifiers inthe patient module 24. Although not shown, it will be appreciated thatany of a variety of electrodes can be employed, including but notlimited to needle electrodes. The EMG responses measured via the EMGharness 26 provide a quantitative measure of the nerve depolarizationcaused by the electrical stimulus. By way of example, the placement ofEMG electrodes 27 may be undertaken according to the manner shown inTable 1 below for spinal surgery:

TABLE 1 Color Channel ID Myotome Spinal Level Blue Right 1 Right VastusMedialis L2, L3, L4 Violet Right 2 Right Tibialis Anterior L4, L5 GreyRight 3 Right Biceps Femoris L5, S1, S2 White Right 4 Right Gastroc.Medial S1, S2 Red Left 1 Left Vastus Medialis L2, L3, L4 Orange Left 2Left Tibialis Anterior L4, L5 Yellow Left 3 Left Biceps Femoris L5, S1,S2 Green Left 4 Left Gastroc. Medial S1, S2

With reference again to FIGS. 1-2, the surgical system 20 performspedicle integrity assessments via, by way of example only, the use ofpedicle testing accessories 30 in combination with the handle assembly36. More specifically, upon pressing the button on the screw test handle36, the software will execute a testing algorithm to apply a stimulationcurrent to the particular target (i.e. pilot hole formation instrumentsand/or preparation instruments and/or the pedicle screw introductioninstruments), setting in motion the pedicle integrity assessmentfunction of the present invention. The pedicle integrity assessmentfeatures of the present invention may include, by way of example only,an “Actual” mode (FIGS. 29-30) for displaying the actual stimulationthreshold 91 measured for a given myotome, as well as a “Relative” mode(FIGS. 31-32) for displaying the difference 92 between a baselinestimulation threshold assessment 93 of a bare nerve root and an actualstimulation threshold assessment 91 for a given myotome. In either case,the surgical accessory label 84 displays the word “SCREW TEST” to denoteuse of the pedicle testing assembly 36 for performing pedicle integrityassessments. The screw test algorithm according to the present inventionpreferably determines the depolarization (threshold) current for allresponding EMG channels. In one embodiment, the EMG channel tabs 82 maybe configured such that the EMG channel having the lowest stimulationthreshold will be automatically enlarged and/or highlighted and/orcolored (EMG channel tab R3 as shown in FIG. 29) to clearly indicatethis fact to the user. As shown in FIG. 25, this feature may beoverridden by manually selecting another EMG channel tab (such as EMGchannel tab R1 in FIG. 30) by touching the particular EMG channel tab 82on the touch screen display 40. In this instance, a warning symbol 94may be provided next to the EMG channel tab having the loweststimulation threshold (once again, EMG channel tab R3 in FIG. 29) toinform the user that the stimulation threshold 91 is not the loweststimulation threshold.

Any number of the above-identified indicia (such as the baselinestimulation 93, actual stimulation 91, difference 92, and EMG channeltabs 82) may be color-coded to indicate general safety ranges (i.e.“green” for a range of stimulation thresholds above a predetermined safevalue, “red” for range of stimulation thresholds below a predeterminedunsafe value, and “yellow” for the range of stimulation thresholds inbetween the predetermined safe and unsafe values—designating caution).In one embodiment, “green” denotes a stimulation threshold range of 9milliamps (mA) or greater, “yellow” denotes a stimulation thresholdrange of 6-8 mA, and “red” denotes a stimulation threshold range of 6 mAor below. By providing this information graphically, a surgeon mayquickly and easily test to determine if the integrity of a pedicle hasbeen breached or otherwise compromised, such as may result due to theformation and/or preparation of a pedicle screw hole and/or introductionof a pedicle screw. More specifically, if after stimulating the screwhole (during formation and/or preparation) and/or pedicle screw itself(during introduction) the stimulation threshold is: (a) at or below 6mA, the threshold display 40 will illuminate “red” and thus indicate tothe surgeon that a breach is likely; (b) between 6 and 8 mA, thethreshold display 40 will illuminate “yellow” and thus indicate to thesurgeon that a breach is possible; and/or (c) at or above 8 mA, thethreshold display 40 will illuminate “green” and thus indicate to thesurgeon that a breach is unlikely. If a breach is possible or likely(that is, “yellow” or “red”), the surgeon may choose to withdraw thepedicle screw and redirect it along a different trajectory to ensure thepedicle screw no longer breaches (or comes close to breaching) themedial wall of the pedicle.

While this invention has been described in terms of a best mode forachieving this invention's objectives, it will be appreciated by thoseskilled in the art that variations may be accomplished in view of theseteachings without deviating from the spirit or scope of the presentinvention. For example, the present invention may be implemented usingany combination of computer programming software, firmware or hardware.As a preparatory step to practicing the invention or constructing anapparatus according to the invention, the computer programming code(whether software or firmware) according to the invention will typicallybe stored in one or more machine readable storage mediums such as fixed(hard) drives, diskettes, optical disks, magnetic tape, semiconductormemories such as ROMs, PROMs, etc., thereby making an article ofmanufacture in accordance with the invention. The article of manufacturecontaining the computer programming code is used by either executing thecode directly from the storage device, by copying the code from thestorage device into another storage device such as a hard disk, RAM,etc. or by transmitting the code on a network for remote execution. Ascan be envisioned by one of skill in the art, many differentcombinations of the above may be used and accordingly the presentinvention is not limited by the scope of the appended claims.

1. A method of delivering stimulation signals to patient tissue duringsurgery for monitoring neuromuscular evoked potentials, comprising thesteps of: (a) attaching an electrical coupling mechanism having aplunger with an electrical contact surface electrically coupled to astimulation source to a surgical instrument, (b) advancing the surgicalinstrument into the patient tissue; (b) applying a stimulation signalthrough the electrical coupling mechanism to the surgical instrument;and (c) monitoring to assess whether nerves are innervating as a resultof the step of applying said stimulation signal.
 2. The method of claim1, wherein the plunger is enclosed within a housing.
 3. The method ofclaim 2, wherein the plunger is generally cylindrical in shape.
 4. Themethod of claim 2, wherein the plunger is spring-loaded relative to thehousing.
 5. The method of claim 4, wherein the plunger is biased in aclosed position.
 6. The method of claim 5, wherein the housing includesa distal arm situated opposite the electrical contact surface.
 7. Themethod of claim 4, wherein the electrical contact surface is generallyarcuate.
 8. The method of claim 5, wherein the electrical couplingsurface is coupled to an electrical cable that is coupled to thestimulation source.
 9. The method of claim 8, wherein pulling on thecable moves the plunger to an open position.
 10. The method of claim 1,wherein monitoring to assess whether nerves are innervating as a resultof the step of applying the stimulation signal includes detectingvoltage responses of a predetermined magnitude with sensors positionedon or within muscles associated with said nerves.
 11. The method ofclaim 10, wherein the stimulation source and the sensors arecommunicatively linked to a control unit.
 12. The method of claim 11,wherein the control unit identifies a relationship between thestimulation signal applied and the voltage response of predeterminedmagnitude.
 13. The method of claim 12, wherein the relationship is thelowest stimulation amplitude required to evoke the voltage response ofpredetermined magnitude.
 14. The method of claim 13, wherein saidrelationship provides an indication of pedicle integrity.
 15. The methodof claim 1, wherein said surgical instrument is configured for at leastone of pedicle hole formation, pedicle hole preparation, and pediclescrew insertion.
 16. A system for monitoring electromyographic (EMG)evoked potentials during surgery, comprising: a plurality of sensorsthat detect EMG responses from muscles associated with nerves locatedproximate to a surgical target site; an electrical coupling mechanismhaving a plunger with an electrical contact surface electrically coupledto a stimulation source and coupleable to a surgical instrument todeliver stimulation signals from the stimulation source to the surgicalinstrument; and a control unit in communication with the plurality ofsensors and the stimulation source, the control unit being operable to(a) direct transmission of the stimulation signals; (b) receive voltageresponses from the plurality of sensors, and (c) determine whether anyof the nerves are innervating as a result of the step of applying thestimulation signals to the surgical instrument.
 17. The system of claim16, wherein the surgical instrument is configured for at least one ofpedicle hole formation, pedicle hole preparation, and pedicle screwinsertion.
 18. The system of claim 16, wherein the plunger is enclosedwithin a housing and spring-loaded relative to the housing.
 19. Thesystem of claim 16, wherein the electrical contact surface is generallyarcuate.
 20. A system for performing pedicle integrity assessments,comprising: a surgical instrument for use in at least one of pediclehole formation, hole preparation, and pedicle screw insertion at apedicle target site, said pedicle target site in the general location ofneural structures; an insulation member through which said surgicalinstrument is passed comprising a flexible hollow elongated sheathhaving a proximal end, a distal end, an interior lumen, and asubstantially enclosed seal tip attached to said distal end of saidhollow elongated member, wherein said seal tip includes an expandableaperture for receiving said surgical instrument; a sensor configured todetect a voltage response from muscles associated with said neuralstructures; and a control unit coupled to said medical instrument andthe sensor, the control unit being configured to (a) transmit astimulation signal to said surgical instrument, (b) receive a voltageresponse from the sensor, and (c) determine whether said neuralstructures are innervating as a result of the step of applying saidapplication of stimulation signal to said surgical instrument.